How to encrypt e-books

I've been using Hikashop for about six years. By now I know it very well. One of my big clients operates an online bookstore. They have a growing selection of e-books. We are encrypting them via a plugin called Haihaisoft. It doesn't work as often as it does. Lots of user complaints that I have to field. We went with this company originally because Hikashop (last time I inquired) didn't offer an encryption plugin for digital goods. The customer doesn't want these expensive books duplicated and circulated on the black market. There are a couple speciality components which can handle this. But we prefer to not add another checkout system if we don't need to. We want to keep the e-book section in Hikashop because it has such great order-tracking features. And the client is well used to them. So please let me know, and please point me to any relevant documentation.
-Jim Gribble

Thank you, Jerome. Are you saying you could implement one of these solutions for us on a custom basis? I would like to hear more about that. We are not happy with Haihaisoft plugin. I would like to obtain ballpark cost estimate as well as understand how such a system work for admin and for end user.
-Jim Gribble

Thank you, Jerome. I will send you a contact e-mail with our requirements. To add a plugin like to Hikashop would be super awesome. Maybe our project can move you guys toward that goal.
-Jim Gribble